FRONT-GILLED GROUP 



STARFISH WITH Thyca ; a. Shell of latter enlarged. 



along on the sole of the foot like most other gastropods. In the family of the naticas 



(Naticida), which is numerous in both recent and fossil species, the shells are more 



or less globular or ear 



shaped, and the an- 



imals remarkable for 



the enormous develop- 



ment of the foot, which 



is used as a plow to 



drive through the sand 



in search of other mol- 



lusks mostly bivalves 



upon which they 



feed. The foot is so 



large as almost to 



conceal the shell, and 



the front part of it 



(the propodium} hides 



the head, which has no 



visible eyes. The 



shells are quite closed 



in some of the groups by a paucispiral operculum, which is either horny or shelly. 



Naticas are found in all parts of the globe. The largest living species (Lunatia 



heros) occurs on the eastern coast of North 

 America, but some of the more highly 

 colored forms are found in tropical seas. 

 About half a dozen species occur in Britain. 

 The spawn cases are curious objects, resem- 

 bling a broad sandy strap coiled round 

 like a horseshoe. 



SECTION PTENOGIvOSSA 



In this unimportant group of Pectini- 

 branchs which includes only the families 

 lanthinidce and Scalariidce the radula 

 consists of numerous rows of pointed 

 teeth, arranged in cross series forming an 

 angle in the middle. There is no central 

 or rhachidian tooth. The violet sea snails 

 {lantkfaidai} have thin trochiform shells, 

 adapted for a pelagic life. These are 

 mostly of a violet color, the base or 

 underside, which is turned upward and 

 VIOLET SEA SNAIL (lanthina) WITH ITS FLOAT, exposed to the light when the animal is 



^^^M &?? '"Z at the surface of the sea, being 

 /. Float ; p. Foot ; t. Head. 



211 







more deeply tinted than the rest of the 



